Conventional cellular telephone architectures must accommodate a wide range of path loss between a cellular handset (user equipment) and a servicing base station, depending on various factors, such as proximity to the base station, radio frequency (RF) obstructions, weather, and the like. As a result, RF power output required from the handset varies widely. A common characteristic of RF power amplifiers handsets, for example, is that they achieve maximum DC to RF power conversion efficiency at the upper limit of power output capability. As the RF power is reduced below this point, efficiency drops, in most cases, dramatically. Since the specific case path loss is usually less than its maximum value, the handset power amplifier is often at reduced power levels where the efficiency is compromised, which may waste battery energy beyond what is theoretically necessary. As handset technology matures in terms of sophistication and functionality, there is an increasing need to conserve battery energy at all points, including the power amplifier.
Conventional design approaches that address the need for greater efficiency in the power amplifier include envelope tracking (ET), average power tracking (APT), multiple mode, and multipath power amplifiers, for example. ET and APT involve adaptation of the power amplifier power supply voltage in a way that conforms with output power demand, so that energy is conserved. Multiple mode power amplifiers allow operating point current in the power amplifier transistors to be optimized for each of several different output power ranges. Multipath power amplifiers have partially separated RF paths for each of several different output power ranges.
Generally, a multipath power amplifier has a high power path and a low power path. FIG. 1 is a simplified block diagram showing an example of a conventional multipath power amplifier. Referring to FIG. 1, multipath power amplifier device 100 includes high power path 110 and low power path 130. The high power path 110 includes first coupling network 111, high power amplifier 112 and output matching network 113 connected in series between input port 101 and output port 102. The high power path 110 is a complete amplifier capable of producing the highest output power demanded of the application. The low power path 120 includes all or part of a second power amplifier, which is designed for lower output power. The low power path 130 includes second coupling network 131, low power amplifier 132, third coupling network 133, and the output matching network 113 connected in series between the input port 101 and the output port 102.
The low power amplifier 132 is coupled into the system using any of a number of possible passive and active coupling networks that allow it to access the input port 101 and the output port 102 of the multipath power amplifier device 100. FIG. 1 is an example that shows the low power path 130 sharing the output matching network 113 with the high power path 110. Input power of the input signal is coupled to the low power amplifier 132 through the second coupling network 131, and output power is coupled into the output matching network 113 through the third coupling network 133. The third coupling network 133 is typically an additional output matching network that transforms the load into impedance higher than the impedance presented to the high power amplifier 112, thus allowing the low power amplifier 132 to operate at lower output power while maintaining high efficiency.
Depending on the power level required, one of the high power amplifier 112 or the low power amplifier 132 will be biased On, and the other of the high power amplifier 112 or the low power amplifier 132 will be biased Off. Since the low power amplifier 132 is optimized for low power, it is highly efficient at low power levels at which the high power amplifier 132 would be very energy wasteful. Thus, conventional multipath power amplifiers, such as the multipath power amplifier device 100, generally offer higher efficiency at low power levels than traditional single path power amplifiers, all else being equal.
However, the high and low power paths 110 and 120 are coupled together at all times. An optimal alignment would present an ideal load condition to high power amplifier 112 when it is in use (On), and present a separate ideal load condition to low power amplifier 132 when it is in use, thus achieving maximum possible efficiency in each state. In conventional systems, however, this is not possible because the third coupling network 133 acts to detune the load impedance to high power amplifier 112 in the high power state. Stated differently, the optimal tuning of the third coupling network 133 for maximizing efficiency in the low power state is not tolerated in the high power state. This is because the third coupling network 133 and the low power amplifier 132 load the high power amplifier 112 differently, depending on whether the low power amplifier 132 is biased On or Off. The best practical alignment requires a compromise that leaves either or both high and low power operating states at sub-optimal performance. Generally, performance of such conventional multipath power amplifiers is worse in each of the high and low power states than counterpart single path power amplifiers. In addition, design and bench tuning are complicated by the tight interaction between the high power path 110 and low power path 130. As a result, the design optimization process is recursive and highly iterative in nature.
A similar problem can occur at the input port 101, where the input load impedance of each of the high power path 110 and low power path 130 depends on whether the corresponding high power amplifier 112 and low power amplifier 132 is biased On or Off. When the high and low power paths 110 and 130 are coupled together at the common input port 101, the input impedance depends on which state is enabled. Thus, it is difficult to achieve uniform input impedance across both (all) states of the multipath power amplifier device 100.